A conventional spark plug used for igniting a combustion engine, such as a gasoline engine for automobiles is comprised of a center electrode, an insulator accommodating the center electrode therein, a cylindrical metal shell accommodating the insulator therein and a ground electrode in which a base end thereof is joined to a front end portion of the metal shell. A male screw portion is formed around an outer circumference face of the metal shell. An outwardly projecting, annular gasket receiving portion is formed on a rear end side of the male screw portion.
A screw hole having a female screw portion is formed in an engine cylinder head. The spark plug is mounted on the engine by screwing the male screw portion into the female screw hole in the engine cylinder head. A portion called a thread neck is provided between a rear end side of the male screw portion of the metal shell and the gasket receiving portion. An annular gasket is provided around the thread neck. With screwing the male screw portion into the screw hole, the gasket is compressed and crushed between the gasket receiving portion and an opening circumference edge portion of the female screw hole to thereby provide a seal between the screw hole and the gasket receive portion.
As a common gasket, it has been known that a gasket formed such that a ring-shaped thin metal plate is radially bent with a special die into, for example, a generally “S” shape in a cross-sectional view, which is so to speak a hollow shape. Such a gasket is subjected to a predetermined nail forming process after being fitted around the thread neck to thereby form a plurality of nail portions (e.g., three nails) projecting in an inner circumference direction. Thus, in the conventional gasket, the nail portion is formed after the gasket is fitted around the thread neck so as to prevent the gasket from falling out from the thread neck (See, for example, Japanese Laid-Open Utility Model Application No. Sho 59-39894).
In recent years, automation has been advanced in various industrial fields. Spark plugs are also automatically mounted on engines using a robot. More particularly, a large number of spark plugs where gaskets are provided are tidily allocated in support holes of a pallet, and a robot picks up a spark plug from the pallet and conveys it to an engine. Then, the robot tightly screws a male screw portion of the spark plug into a screw hole of an engine cylinder head. In addition, when the spark plug is automatically mounted on the engine cylinder head, the spark plug is supported in a state where the gasket is in contact with a circumference of the support hole of the pallet and extends in a vertical direction.
However, the conventional spark plug tends to have a problem that a nail portion of the gasket is likely to enter into between adjacent thread ridges of the male screw portion, and also the male screw portion bites into the nail portion. When this problem occurs, the gasket inclines with respect to an axial center of the spark plug when mounted. Therefore, in this case, the spark plug having the inclined gasket with respect to the circumference of the support hole of the pallet is supported at the time of the automatic mounting operation. That is, the spark plug is supported in an inclined state. Therefore, there is a possibility that the robot may incorrectly operate a removal control or may judge that a supporting position of the spark plug is not in a scheduled location. As a result, the mounting operation of the spark plug on the engine may be hampered.
In addition, the problem that the male screw portion bites into the gasket as mentioned above does not only occur to the spark plug where the gasket is provided, but also occurs to automotive components, such as sensors, where various kinds of gaskets are provided.
The present invention has been accomplished in light of the above-mentioned problems, and an object of the present invention is to provide a gasket provided around a thread neck of an automotive component. The invention prevents the gasket from falling out, as well as effectively preventing a male screw portion from biting into the gasket.